1. Field of the Invention
The present invention relates to an operating and evaluation circuit of an insect sensor, which includes a resistor that is dependent on insect infestation, whereby the circuit, upon stimulation via a transponder, generates a current flow through the resistor, detects a change in the resistor as a change in voltage, and compares it with a predetermined threshold.
2. Description of the Background Art
In some parts of the world, there is an increased risk of damage to buildings due to the eating of wood by termites living underground. Wooden structures in the USA, Australia, and New Zealand, but also in southern Europe are particularly affected by this. RFID tags (RFID=radio frequency identification) with external resistor paper strips are buried in soil to determine whether a certain area is threatened by such species. Attracted by the bait-impregnated strips, the possibly present termites attack the resistor paper strip and consume it gradually so that its electrical resistance increases. If a reader is brought sufficiently close to the sensor, it supplies the sensor with energy for a resistance measurement according to a transponder principle. Resistance values lying above a threshold are signaled to the reader by the operating and evaluation circuit and indicate insect infestation, particularly termite infestation.
Conventional operating and evaluation circuits typically use a voltage divider with the RFID resistor and another resistor. A center tap of the voltage divider controls a transistor whose working current path lies between an earth potential and a terminal, at which a reader generates a supply potential via the transponder. A collector terminal of the transistor in the current-free state is pulled to the ground potential via a pull-down resistor.
In the undamaged state without insect infestation, the RFID resistor strip has a relatively small resistance, so that with a build up in the supply potential via the transistor most of the voltage applied at the voltage divider declines across the other resistor. There is only a small potential then at the center tap, which is not sufficient to control the transistor conductively. The working current path is therefore virtually current-less, so that the sensor potential corresponds approximately to the ground potential because of the pull-down resistor.
With insect infestation, the resistance of the RFID resistor strip increases. As a result, at the center tap of the voltage divider an increased voltage occurs, which controls the transistor conductively, which enables a current over its working current line and thereby a voltage drop across the pull-down resistor. The voltage drop is detected by a threshold detector and a value exceeding the threshold is signaled to the reader via the transponder connection.
In the conventional circuit, the additional resistor, transistor, and a resonance capacitor of the transponder are disposed outside the transponder housing on a board of the insect sensor. A current of 4 to 10 mA must be provided to achieve an open base-emitter voltage of about 0.7 V in the transistor to supply the external components in the conventional circuit and at resistance values of the resistor strip between 15 kΩ in the new state and about 100 kΩ for the so-called trip point, which characterizes insect infestation. Because this current must be induced via the transponder, the high current requirement goes hand in hand with a high damping of the input resonant circuit of the circuit, which interferes with the reader range. The high current requirement therefore reduces the distance up to which stimulation and reading of the circuit with a transponder-reader is possible.